There are numerous general technical applications with the need for extremely miniaturised motors that are able to make controlled fine-positioning. Among these, portable consumer devices, such as e.g. cameras, have additional demands for low power consumption, low weight and price. The required motion is typically linear but often a rotating motor combined with a motion-converting mechanism such as a lead-screw is used. The motion range is often in the order of millimeters. The demands described above are difficult to satisfy simultaneously by one and the same motor design.
Generally, the use of a solid actuator material and some type of magnification mechanism appear to be the best solution for the desired miniature motors. A solid actuator such as a PZT element has a very high energy density and the size of a motor could therefore be made very small. Typically, existing actuator materials with high energy density are not able to change its shape more than a tenth of a percent, which makes it difficult to make an optimised construction with small external dimensions. Many different solutions have been proposed in prior art. A few typical examples are discussed below.
In U.S. Pat. No. 6,373,170, two separate actuator blocks inclined with a given angle relative to each other uses an interlinking drive pad to drive a rail. The actuators are operating with a so-called 33-actuation. The overall V-shaped unit extends perpendicular to the moving rail. The two actuators are driven in a longitudinal mechanical resonance with a phase shift between the two actuators giving an elliptical trajectory of the drive pad. When making the components small, in the order of a few millimeters, longitudinal resonance frequencies will occur in the vicinity of the MHz range. In many applications, such frequencies will give rise to electromagnetic interference and will not be accepted. It is also practically difficult to operate devices at such frequencies. Also, the geometrical design, using a non-negligible angle between the actuator block and the rail makes the assembly unsuitable for miniaturised motors.
In U.S. Pat. No. 5,453,653, one actuator plate with several electrodes operates with 31-actuation with two different resonance modes at the same time. This creates an elliptical trajectory if there is a phase shift between the applied voltage signals. Since one longitudinal resonance mode is used also here, similar objections as in the previous disclosure exist also here. In this case they become even more severe, since the geometry is further restricted by the demand that a bending mode resonance should occur at approximately the same frequency. Furthermore, the perpendicular geometry in relation to the body to be moved is unsuitable for small-size applications.
One component that has been used in numerous applications is a piezoelectric bimorph element since a high internal motion magnification can be achieved in a bending mode. A motor intended for applications, which demand extremely small sizes, was constructed and presented in the Swedish patent SE9300305-1. Rotation or linear translation is performed by stepwise motion with bimorph elements in direct contact with the object to be moved. In this invention, the bimorph elements are driven in such a way that the contact point of the bimorph element moves in two dimensions, i.e. the bimorph is used both in the bending and the longitudinal direction.
In the German patent DD 143 682, a piezoelectric stick-slip motor is disclosed. Two bimorphs consisting of a steel/piezoelectric element sandwich are connected by an intermediate passive steel part. A lever connected to the passive part act on a wheel to be driven, The in-phase bending of the bimorphs is used for driving the wheel while the out-of-phase bending of the bimorphs rotates the passive part and varies the contact pressure on the wheel. The construction is however, unsuitable for applications where the space around the body to be moved is limited.
In the U.S. Pat. No. 5,089,740 a driving mechanism based on bimorphs is disclosed. A complex gate-shaped arrangement of bimorphs with coupled motion patterns is presented, which has a relatively high rigidity. However, the entire arrangement is space consuming, and using parts of the arrangement will remove the advantages of the coupled motions.
U.S. Pat. No. 6,392,328 discloses an arrangement with one actuator beam with several electrodes. The beam operates with an 31-actuation at two different resonant modes that create elliptical trajectories of two drive pads in contact with a rail oriented parallel to the beam. The beam is attached to a support in the central part of the beam. Also here a longitudinal resonance is used, which gives the disadvantages mentioned above. Furthermore, the use of more than one drive pad restricts the maximum step length in relation to the length of the actuator arrangement.
In U.S. Pat. No. 6,437,485, an actuator arrangement suitable for limited lateral space around a body to be driven is disclosed. One actuator beam, comprising several electrodes and arranged substantially parallel to the body, operates with 31-actuation close to a fundamental resonance frequency. The beam is supported against a base substantially at the ends, and a single drive pad is arranged in the centre of the beam. An asymmetric supply of voltages to the two sides of the beam and a frequency slightly off-resonance are used to achieve a two-dimensional trajectory of the drive pad. The advantages with this arrangement are many. It requires very small lateral space in the vicinity of the body, the use of a single drive pad enables long strokes and the near-resonance operation reduces electronics requirements. However, it is relatively difficult to achieve a sufficient efficiency.